1. Field of the Invention
The invention relates to silicon seed particles for producing polycrystalline silicon granules in a fluidized-bed reactor.
2. Description of the Related Art
Polycrystalline silicon granules are an alternative to the polysilicon produced in the Siemens process. Whereas the polysilicon in the Siemens process is produced as a cylindrical silicon rod which, before further processing thereof, must be time-consumingly and expensively comminuted to form what is termed chip poly and optionally again purified, polycrystalline silicon granules have bulk goods properties and can be directly used as raw material, eg for single-crystal production for the photovoltaics and electronics industry.
Polycrystalline silicon granules are produced in a fluidized-bed reactor. This is achieved by fluidizing silicon particles by means of a gas stream in a fluidized bed, wherein the fluidized bed is heated up to high temperatures via a heating device. By adding a silicon-containing reaction gas, a thermochemical reaction proceeds, wherein elemental silicon is deposited on the hot particle surface. The individual particles grow in diameter. By regular takeoff of grown particles and addition of smaller silicon seed particles, the process can be operated continuously, with all of the advantages associated therewith. As silicon-containing reagent gas, silicon-halogen compounds (eg chlorosilanes or bromosilanes), monosilane (SiH4), and also mixtures of these gases with hydrogen are described. Such deposition processes and devices therefor are known, for example, from U.S. Pat. No. 4,786,477 A.
For producing high-purity polycrystalline silicon granules, silicon seed particles are necessary.
Gas jet mills, e.g. from U.S. Pat. No. 7,490,785 B2, are known for the production of such silicon seed particles. Such devices are suitable for producing silicon seed particles of a size of 50 μm to 1000 μm from silicon granules of a size of 300 μm to 5000 μm, and comprise a vertically arranged jet chamber having a cylindrical cross section having a jet nozzle at the bottom of the jet chamber, through which a milling gas stream can be introduced into the jet chamber, a counterflow gravity sifter directly following the jet chamber, and an intake for silicon granules, wherein the jet chamber is sufficiently long for an expansion of the milling gas stream to the cross section of the jet chamber, and the jet chamber has a smaller flow cross sectional area than the counterflow gravity sifter. The silicon granules are comminuted, in that a fluidized bed of high solids concentration forms from the silicon granules in the milling zone, individual silicon particles of the silicon granules in the cylindrical jet chamber in which only a low solids concentration is present are accelerated by a high-velocity milling gas stream and impact onto the fluidized bed of high solids concentration, wherein silicon granules and silicon particles shatter.
In one embodiment, parts of the device coming into contact with the silicon particles consist of an external metallic sheath having an inner wall which is provided with a coating. As coating, silicon in monocrystalline or polycrystalline form, or a plastic, is used.
However, it has been found that jet nozzles with coatings made of silicon or plastic are subject to high abrasion, in particular at the exit region of the nozzle.
US 2011/073689 A2 discloses a process for generating fine particles by means of a jet mill, wherein the milling gas has a pressure of <=4 bar (absolute) and a temperature of below 100° C. Likewise, a process for generating fine particles by means of a jet mill, using compressed gases as milling gas, is described, in which the milling gas has a pressure of <=4 bar (absolute) and a temperature of below 100° C., and in which, during the milling, a coating medium or doping medium is added in such a manner that at least parts of the surface of the particle are coated with the coating medium or doping medium.
If silicon seed particles having particle sizes of greater than 1250 μm are to be produced, the abovedescribed jet mills are not suitable. However, for the production of silicon seed particles of such a size, use can be made of roller crushers. JP 57-067019 A discloses the production of silicon seed particles by comminuting polycrystalline silicon in a roller crusher and subsequent fractionation by sieving. The rollers are prepared from high-purity silicon.
In this case, however, high abrasion of the silicon coating on the rollers occurs, in such a manner that only short running times of the machine are possible until the rollers must be replaced. Economic production of silicon seed particles is not possible thereby.
U.S. Pat. No. 7,549,600 B2 discloses a process for producing fine silicon fragments by comminution in a crusher, classifying the fine fragments, wherein a part of the crushed material having an edge length which is less than or equal to the maximum edge length of the desired fine silicon fragments (fraction 1) is collected in a collection container 1, and the part of the crushed material having an edge length which is greater than the edge length of the desired fine silicon fragments (fraction 2) is likewise collected. In one embodiment, from fraction 1, a part of the fine fragments which has an edge length which is less than the minimum length of the desired fine silicon particles, is extracted by sorting and collected (fraction 3). The resultant fractions 1 and 3 can be used as seed particles for depositing polycrystalline silicon in a fluidized-bed process. The crushing tools have a surface made of a hard metal (particularly preferably tungsten carbide in a cobalt matrix) or of silicon.
In this case, however, unwanted contamination of the silicon with B, C, Zn, Ti, Mg, W, Fe, Co, Sb and Zr occurs.
U.S. Pat. No. 5,637,815 A discloses a nozzle made of sintered SiC for use in fluidized-bed reactors, wherein reagents are introduced through the nozzle and are mixed or dispersed.
DE 100 520 21 B4 discloses a water jet cutter high-pressure nozzle, consisting of sintered WC powder. A water jet cutting machine is a machine tool for separating materials such as plastics, metals, leather and stones by means of a high-pressure water jet.
The objective of the invention resulted from the problems described.